A mobile radio communication system, such as a UMTS (Universal Mobile Telecommunication System) type system, includes a mobile radio communication network communicating with mobile terminals or UEs (User Equipments) and with external networks (not specifically shown). The mobile radio communication network includes: a radio access network or UTRAN (UMTS Terrestrial Radio Access Network) and a core network CN.
Third generation systems, in particular of UMTS Universal Mobile Telecommunication System) type, use a radio access technology of W-CDMA (Wideband-Code Division Multiple Access) type. The UTRAN includes base stations (each of which is called a Node B) and base station controllers or radio network controllers (RNC). Each base station provides radio coverage for one or more cell areas. The UTRAN communicates on the one hand with the mobile terminals UE, via an interface called the “Uu” interface (or radio interface), and on the other hand with the CN via an interface called the “Iu” interface. In the UTRAN, the Node B base stations communicate with the base station controllers RNC via an interface called the “Iub” interface, and an interface called the “Iur” interface can further be provided between RNCs.
Certain aspects of radio resource control used in the UTRAN are outlined briefly here. For a more complete description, see for example the 3GPP Technical Specification TS 25.331 defining the RRC (Radio Resource Control) protocol and published by the 3GPP (3rd Generation Partnership Project). For optimized management of the radio resources, the RNC executes certain mobility management functions. Mobility management algorithms have therefore been defined in the UTRAN, based on a system of states called RRC states and transitions between those states.
There are two modes for a UE: idle mode, in which there is no connection between the UE and the UTRAN, and the UTRAN has no information on the location of the LIE, and connected mode, in which there is a connection between the UE and the UTRAN, and the UTRAN holds information on the location of the UE. In the connected mode, various possible states of a UE are distinguished: a state called the Cell_DCH state (where DCH stands for Dedicated Channel), in which the UTRAN knows that the UE is located in one or more cells called active cells, the UE effecting radio measurements and reporting the results of those measurements to the network, a state called the Cell_FACH state (where FACH stands for Forward Access Channel), in which the UTRAN knows that the UE is located in a cell, the UE effecting radio measurements and itself effecting cell reselection on the basis of the results of these measurements and informing the network thereof by means of a cell update procedure, a state called the Cell_PCH state (where PCH stands for Paging Channel), in which the UE can be contacted only by means of a paging procedure, and the location of the terminal is known at the cell level, the UE effecting radio measurements and itself effecting cell reselection on the basis of the results of those measurements and informing the network thereof by means of a cell update procedure, a state called the URA_PCH state (where URA stands for UTRAN Registration Area and PCH stands for Paging Channel), in which the UE can be contacted only via a paging procedure and the location of the terminal is known at the set of cells or URA level, the UE effecting radio measurements and itself effecting cell reselection on the basis of the results of those measurements and informing the network thereof via a URA update procedure.
The radio measurements effected by the UE are controlled by the network (UTRAN). The UTRAN can therefore send various radio measurement control information to the UE, e.g., by broadcasting system information and/or by transmitting a dedicated signaling message called a Measurement Control message.
TS 25.215 relates to physical layer (L1) measurements and outlines a toolbox of measurement abilities for the UE and the UMTS terrestrial radio access network (UTRAN). These measurements can be differentiated in various reported measurement types: intra-frequency measurements (measurements effected on the same frequency as that used by the active cell or cells with which a UE is currently connected), inter-frequency measurements (measurements effected on a frequency different from that used by the active cell or cells), inter-RAT measurements, where RAT stands for “radio access technology” (or measurements effected on cells of another system, for example the GSM (Global System for Mobile communications)), using a radio access technology different from that used for the active cell or cells, traffic volume, quality, and UE internal measurements. The measurements are distinguished between measurements in the UE (the messages will be described in the RRC Protocol or MAC Protocol) and measurements in the UTRAN (the messages will be described in the NBAP and the Frame Protocol).
To initiate a specific UE measurement, the UTRAN transmits a “measurement control message” to the UE including a measurement ID and type, a command (setup, modify, release), the measurement objects and quantity, the reporting quantities, criteria (periodical/event-triggered) and mode (acknowledged/unacknowledged). The results of the measurements are sent back to the UTRAN in measurement report messages either periodically, after some triggering event, or using a combination of the two. When the reporting criteria are fulfilled, the UE answers with a “measurement report message” to the UTRAN including the measurement ID and the results.
Intra-frequency cell measurements are based on measurements on the Received Signal Code Power (RSCP), which is the received power on one code measured on the Primary Common Pilot CHannel (CPICH), and/or on the energy per chip divided by power density in the intra-frequency band Ec/No for the CPICH. Depending on whether these measurements fulfill some condition(s), the UE sends the UTRAN report with the current intra-frequency cell measurements.
A UE may perform soft handover (SHO) to improve reception quality in a peripheral area of a cell. In SHO, the UE communicates with multiple radio base stations (or cells/sectors) with radio links connected therebetween. In general, the radio link between the mobile station and each base station is called a “branch,” and a set of the multiple base stations/cells having communications made through the radio links in SHO is called an “active set.” In addition, “branch addition” (referred to as reporting event 1A in TS 25.331) indicates an event where a SHO link with a new radio base station cell is added to the mobile's active set, and “branch exclusion” (referred to as reporting event 1B in TS 25.331) indicates an event in which a radio base station cell already communicating with the UE through a radio link is excluded from the active set. Moreover, “branch replacement” (referred to as reporting event 1C in TS 25.331) indicates an event in which the radio base station cell having he poorest radio quality in the active set is replaced with a radio base station cell not included in the active set but having better radio quality when the number of radio base station cells in the active set exceeds a predetermined value. Reporting event 1D in TS 25.331 changes the current best cell in the active set to a new best cell.
A UE monitors radio quality (for example, CPICH RSCP, CPICH Ec/N0 and pathloss) between the UE and each of the radio base station cells, makes a determination on branch addition, branch exclusion, branch replacement, and best cell change by using the radio quality, and then reports the determination result to a radio control station. Then, the radio control station makes a control for SHO according to the content reported from the UE
The Time-To-Trigger (TTT) is normally a parameter broadcasted by the network in the System Information Block (SIB). UEs follow the measurement and reporting processes described above. After a measured condition is fulfilled and after TTT has expired, a measurement report is sent to the network. The measurement report is processed by the network, and the network may respond based on the report by sending a command to the UE, e.g., a cell change command. During network processing time, the current source cell signal quality may drop rapidly so that UE is not able to receive data from the network. As a result, the UE would not receive the network commands such as the cell change command. Unfortunately, this may well result in the ongoing call being dropped, which is highly undesirable.